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Tongue Has a Sixth Sense

The human tongue may have a sixth sense—and no, it doesn’t have anything to do with seeing ghosts. Researchers have found that in addition to recognizing sweet, sour, salty, savory, and bitter tastes, our tongues can also pick up on carbohydrates, the nutrients that break down into sugar and form our main source of energy. Past studies have shown that some rodents can distinguish between sugars of different energy densities, while others can still tell carbohydrate and protein solutions apart even when their ability to taste sweetness is lost. A similar ability has been proposed in humans, with research showing that merely having carbohydrates in your mouth can improve physical performance. How this works, however, has been unclear. In the new study, to be published in Appetite, the researchers asked participants to squeeze a sensor held between their right index finger and thumb when shown a visual cue. At the same time, the participants’ tongues were rinsed with one of three different fluids. The first two were artificially sweetened—to identical tastes—but with only one containing carbohydrate; the third, a control, was neither sweet nor carb-loaded. When the carbohydrate solution was used, the researchers observed a 30% increase in activity for the brain areas that control movement and vision. This reaction, they propose, is caused by our mouths reporting that additional energy in the form of carbs is coming. The finding may explain both why diet products are often viewed as not being as satisfying as their real counterparts and why carbohydrate-loaded drinks seem to immediately perk up athletes—even before their bodies can convert the carbs to energy. Learning more about how this “carbohydrate sense” works could lead to the development of artificially sweetened foods, the researchers propose, “as hedonistically rewarding as the real thing.”